GAV wrong in README.md

The "Usage" sections says:
```xml
<groupId>com.github.a-schild</groupId>
<artifactId>nextcloud-java-api</artifactId>
```

While infact it should state (as per pom.xml):
```xml
<groupId>org.aarboard.nextcloud</groupId>
<artifactId>nextcloud-api</artifactId>
```